1. Field of the Invention
This invention relates to the precise measurement of small distances or displacements by means of an optical source and an optical detector.
2. Description of the Prior Art
The precision measurement of small distances of displacements is typically accomplished by resistance-type strain gauges, wherein a resistive element is attached to the structure being measured, and the change in resistance correlates to a change in displacement per unit length. While this is suitable for structures that are relatively unaffected by the force required in the operation of the strain gauge, it would not be suitable for situations where the force required for operation of the strain gauge is large compared to the force deforming the structure. Furthermore, resistance-type strain gauges are permanently deformed if the displacement exceeds a given value and cannot be used for measurements exceeding a certain number of cycles. Other devices have also served as displacement measuring devices, including piezoelectric devices, capacitance devices, or inductance devices. The piezoelectric devices produce a voltage output that is proportional to the strain applied. However, again a force is required on the piezoelectric device for operation. Furthermore, piezoelectric devices typically have a significant hysteresis, making repeatability of measurements difficult.
Capacitance devices typically work by passing a radio frequency current through the parallel plates of a capacitor, which are attached to the objects to be measured. The change in capacitance between the capacitor plates is used as an indication of the displacement of the plates. Inductance position measuring devices typically operate by varying the position of a high permeability core inside of an inductor. While capable of a precise measurement, typically the core has a relatively large mass, limiting the frequency response.
Optical devices have also been used for measuring distances or displacements which overcome certain of the above shortcomings. For very precise measurement, optical interference devices have been used. These typically use a laser or other monochromatic light source to obtain an interference pattern that can be used to determine the displacement of a device. While very accurate, such devices typically require elaborate supporting electronics, especially if rapid readout of a distance or a displacement is required. Various other optical devices have also been used. These include measuring the position of a reflected optical spot on a detector to determine the position of the reflective surface. Two reflective surfaces at right angles have also been used in an optical strain gauge (a "Tuckerman extensometer") wherein one of the surfaces rotates, with the resulting deflection of the light beam being determined by means of a reticle. Also, a bundle of parallel optical fibers has been used to obtain optical displacement measurements; see, for example, U.S. Pat. No. 3,327,584.